This invention relates to the preparation of epoxy resins. In a specific aspect, the invention relates to a process for reacting, in an epoxy "advancement" or "fusion" reaction, a phenolic compound with a low molecular weight epoxy resin to prepare a high molecular weight fusion product. In a further specific aspect, the invention relates to a catalyst useful for such an advancement reaction.
It is known to prepare epoxy resins by reacting a compound having a vicinal epoxide group, such as epichlorohydrin, with bisphenol-A in the presence of sodium hydroxide. To prepare linear, high molecular weight epoxy resins, it is common practice to first prepare an epoxy resin having a relatively low weight per epoxide (WPE) and then to react it, in the presence of a catalytic amount of a suitable catalyst, with a compound containing phenolic hydroxyl groups. Various catalysts have been employed in such fusion reactions, including phosphonium salts such as those disclosed in U.S. Pat. No. 3,447,990 and U.S. Pat. No. 4,438,254, both of which disclose various classes of phosphonium salt fusion catalysts.
Such phosphonium halide catalysts are highly active in promoting the epoxy fusion reaction to produce a high molecular weight epoxy resin. It is often desirable to prepare "precatalyzed" mixtures of the epoxy resin and the fusion catalyst which have good storage stability even at elevated temperatures. The use of some phosphonium halide catalysts results in a precatalyzed system which is inactive after storage, particularly under elevated temperatures. Some phosphonium halide catalysts, as discussed in U.S. Pat. No. 4,438,254, are very water-sensitive, necessitating the careful control of water in the advancement reaction mixture.
It is therefore an object of the invention to provide new fusion catalysts which are highly active in the fusion reaction but which can be included in a precatalyzed epoxy composition having good storage stability and low sensitivity to moisture.